1. Field of the Invention
The present invention relates to a microorganism deposition preventing system for preventing clogging pipes with microorganisms or reducing the heat-exchanging efficiency of a heat exchanger by microorganisms deposited over the heat-exchanging surface of the heat exchanger, and improvements in a microorganism removing method.
2. Description of the Prior Art
While a large amount of industrial water is used for various purposes including cooling and processing in power stations and chemical plants, microorganisms and waterweed deposit in slime in pipes to clog the pipes or over the heat-exchanging surface of a heat exchanger to reduce the heat-exchanging efficiency of the heat exchanger. Prior art system for preventing such biological troubles disclosed in USP 4,453,953 and Japanese Patent Publication (Kokoku) Nos. 55-61984, 61-11882, 61-11883 and 64-3157 pass an ozonous gas having a high ozone concentration intermittently for a short time at a time through a pipe to remove biological slime.
Industrial water, in general, contains several parts per million of organic pollutants and metal ions, which are highly reactive with ozone, and ozone is highly subject to self-decomposition when the hydrogenion activity of industrial water is pH 7 or above. Accordingly, most part of ozone to be mixed in industrial water is consumed in an ozone mixing device, such as an ejector, by reaction with pollutants and metal ions and self-decomposition.
A known microorganism removing method injects chlorine or a chloric chemical into industrial water to prevent the adhesion and propagation of microorganisms by the bactericidal action and weedicidal action of chlorine. In removing microorganisms by such a microorganism removing method, the chlorine concentration of industrial water, in general, is 1 ppm, which needs a large amount of chlorine and increases the operating cost. Furthermore, if industrial water containing chlorine is drained into water for public use, such as rivers or sea, the industrial water will cause environmental pollution. Accordingly, such industrial water must be treated by a wastewater treatment system before draining the same, which, however, is very expensive.
A microorganism removing method proposed to eliminate those disadvantages of the foregoing microorganism removing method uses ozone, which has bactericidal and weedicidal actions higher than those of chlorine and decomposes into harmless oxygen in a relatively short time in water. This microorganism removing method is effective on preventing the adhesion and propagation of microorganisms on pipes for passing cooling water when ozone is supplied continuously into cooling water so that the effective ozone concentration of cooling water is on the order of 0.1 ppm. However, as mentioned above, since ozone decomposes in a relatively short time in water, ozone needs to be supplied into cooling water so that the ozone concentration of cooling water is on the order of 0.5 ppm, which is far greater than the effective ozone concentration. Since the price of ozone is about four times that of chlorine, the use of ozone is economically difficult and the microorganism removing method using ozone has not been widely applied to practical use.
An invention proposed in Japanese Patent Publication (Kokoku) No. 62-10714 to solve those problems is intended to extend the life of ozone in water by supplying an acid, such as carbonic acid, chloric acid or sulfuric acid, from an acid source into cooling water containing ozone supplied intermittently from an ozone source.
An invention proposed in U.S. Pat. No. 4,453,953 and Japanese Patent Laid-open (Kokai) No. 55-61983 provide a microorganism removing method that reduces the necessary quantity of ozone, enhances the economic effect of ozone and prevents environmental pollution by periodically supplying an ozonous gas having a high ozone concentration into water.
Referring to FIG. 1 showing a prior art organic deposition preventing system disclosed in Japanese Patent Publication (Kokoku) No. 62-10714, there are shown an ozonizer 1, an oxygen source 2 from which oxygen is supplied to the ozonizer 1, an ozone adsorber 4, a circulating fan 3 provided on a line connecting the ozonizer 1 to the ozone adsorber 4, a cooling medium supply unit 5 for supplying a cooling medium to the adsorber 4 to cool the same, a heating medium supply unit 6 for supplying a heating medium to the ozone adsorber 4 to heat the same, a water-jet ejector 7 which desorbs ozone from the ozone adsorber 4 under reduced pressure and mixes the desorbed ozone in cooling water, selector valves 8a to 8g, an ejector driving water line 9 connected to the water-jet ejector 7, a cooling water line 10 communicating with the ejector driving water line 9, an ejector driving pump 11 provided on the ejector driving water line 9, a solenoid valve 12, an acid tank 14, and a metering pump 13 for injecting an acid supplied from the acid tank 14 into the cooling water line 10. These components constitute an acid supply system.
The ozone adsorber 4 has an inner cylinder 4a and an outer cylinder 4b surrounding the inner cylinder 4a. The inner cylinder 4a is packed with an ozone adsorbent, such as silicagel, and the space between the inner cylinder 4a and the outer cylinder 4b is filled up with a heat medium, such as ethylene glycol or an alcohol. The circulating fan 3, the ozonizer 1 and the ozone adsorber 4 are arranged in that order to form a circulating system, and the oxygen source 2 is connected to the circulating system.
The operation of the organic deposition preventing system will be described hereinafter. The organic deposition preventing system is capable of ozone adsorbing operation and ozone desorbing operation.
In the ozone adsorbing operation, oxygen is supplied into the circulating system from the oxygen source 2 so that the pressure in the circulating system is constant. Normally, the pressure in the circulating system is 1.5 kg/cm.sup.2. The selector valves 8c and 8d are open. Oxygen is circulated through the circulating system by the circulating fan 3, part of the oxygen flowing through the discharge gaps of the ozonizer 1 is ozonized by silent discharge. An ozonous gas containing ozone is supplied into the ozone adsorber 4. Then, the ozone adsorbent contained in the ozone adsorber 4 adsorbs ozone selectively, and the oxygen is returned through the selector valve 8c to a return line connected to the circulating fan 3. The oxygen source 2 replenishes the circulating system with oxygen of an amount corresponding to the amount of oxygen converted into ozone. The ozone adsorbent is cooled by the cooling medium supplied from the cooling medium supply unit 5 to a temperature of -30.degree. C. or below, because the ozone adsorbing capacity of the ozone adsorbent is greatly dependent on temperature; that is, the ozone adsorbing capacity of the ozone adsorbent increases when the temperature thereof is raised, and decreases when the temperature thereof is lowered.
In the ozone desorbing operation, when the ozone adsorbent contained in the ozone adsorber 4 is nearly saturated with ozone, the ozone desorbing operation is started. During the ozone desorbing operation, the ozonizer 1, the circulating fan 3 and the cooling medium supply unit 5 are stopped, the selector valves 8a, 8b, 8c and 8d are closed, the heating medium supply unit 6, the water-jet ejector 7 are started, and the selector valves 8e, 8f and 8g are opened. Then, the ozone adsorbent is heated by the heat supplied from the heating medium supply unit 6 to promote the desorption of ozone from the ozone adsorbent. Then, the water-jet ejector 7 sucks the ozone from the ozone adsorber 4 under reduced pressure, mixes the ozone with water therein and delivers ozone-containing water into the cooling water line 10 to suppress organic deposition in the cooling water line 10. During the suction of ozone under pressure, the internal pressure of the ozone adsorber 4 is about -70 cmHg.
When the organic deposition preventing system is set for the ozone desorbing operation by closing the selector valves 8a, 8b, 8c and 8d, opening the selector valves 83 and 8f and starting the heating medium supply unit 6, the ejector driving pump 11 is started and the solenoid valve 12 is opened in synchronism with the start of the ejector driving pump 11, the metering pump 13 starts supplying the acid, the selector valve 8g is opened after the metering pump 13 has started, and the organic deposition preventing system starts supplying ozone into the cooling water line 10. After the completion of the ozone desorbing operation, the solenoid valve 12 is closed, the metering pump 13 is stopped to stop supplying the acid, the selector valves 8e, 8f and 8g are closed, the ejector driving pump 11 is stopped, and then the ozone adsorbing operation is started again. Thus, the ozone adsorbing operation and the ozone desorbing operation are repeated alternately.
When ozone is injected in an ozone concentration of 10 ppm into cooling water circulating through the cooling water line 10, the ozone concentration decreased to 1 ppm in two minutes when the hydrogen-ion activity of the cooling water is on the order of pH 8, whereas the ozone concentration decreased to 6.7 ppm and 7.1 ppm in two minutes when the hydrogenion activity of the cooling water was pH 6 and pH 7, respectively. Such a high effective ozone concentration has a high bactericidal effect and hence a high organic deposition preventing effect.
Incidentally, another method of regulating the pH of cooling water uses carbon dioxide gas. FIG. 2 is a flow chart of a prior art pH regulating method using carbon dioxide gas applied to the Oklahoma State Water Purifying Plant, U.S.A. started its operation in July, 1988. This pH regulating method is described in Tomio Deguchi, "Advanced Water Purifying Technology Using Ozone", pp. 286-287. This prior art pH regulating method purifies foul water in a sedimentation basin 21 containing limestone 22, i.e., an alkaline substance, regulates the pH of the purified water (neutralization) by carbon dioxide gas 23, i.e., an acidic substance, and then blows an ozonous gas 24 into the neutralized water.
FIG. 3 shows a microorganism removing system for carrying out a microorganism removing method disclosed in Japanese Patent Laid-open (Kokai) No. 55-61983. Referring to FIG. 3, cooling water supplied to a cooling water line 101 by a water pump 102 is discharged after cooling a heat exchanger 103. An ozonizer 104 produces an ozonous gas of a high ozone concentration by converting oxygen or oxygen contained in air by silent discharge. The ozonous gas containing ozone is supplied through an ozone supply line 105 to a water-jet ejector 106, i.e., an ozone mixing device. The water-jet ejector 106 is driven by water discharged from an ejector driving pump 107 and supplied through a driving water supply line 108 connected to the water-jet ejector 106. The ozone supply line 105 is provided with a solenoid valve 109.
In operation, an ozonous gas is produced by the ozonizer 104 by subjecting oxygen or air to ozonization using silent discharge and, at the same time, the ejector driving pump 107 is driven, the solenoid valve 109 is opened to send the ozonous gas of a high ozone concentration through the ozone supply line 105 to the water-jet ejector 106 connected to the driving water supply line 108, and then the ozonous gas is blown in minute bubbles into cooling water supplied by the water pump 107. The ozonous cooling water having a high ozone concentration containing minute ozone bubbles is mixed in the cooling water flowing through the cooling water line 101. Then, the ozone contained in the cooling water destroys microorganisms adhering to the inner surface of the cooling water line 101 to prevent the cooling water line 101 to be clogged with microorganisms and to prevent the adhesion of organisms to the surface of the heat exchanger 103. The ozonous gas is injected periodically into the cooling water, for example, once a day for five minutes, so that the ozone concentration of the cooling water is maintained at a value in the range of 5 to 10 ppm. After the ozonous gas has been injected into the cooling water for five minutes, the solenoid valve 109 is closed, the ozonizer 104 is stopped, and then the ejector driving pump 107 is stopped.
Periodic injection of such ozonous water having a high ozone concentration into the cooling water removes microorganisms because of the following reasons. Since microorganisms propagate at an exponential rate, the propagation of microorganisms can be prevented by perfectly destroying the microorganisms in the initial stage of propagation by the ozonous water and periodically repeating destroying microorganisms in such a manner. Thus, troubles in the cooling water line attributable to the adhesion of microorganisms to the inner surface of the cooling water line can be prevented and organisms adhering to the surface of the heat exchanger can be removed.
As stated above, the prior art microorganism removing method supplies ozone periodically into the cooling water to maintain the ozone concentration of the cooling water in the range of 5 to 10 ppm. However, cooling water, in general, contains several parts per million of organic pollutants, suspended solids (SS) and inorganic ions including iron ions, which are highly reactive with ozone, and ozone is highly subject to self-decomposition when the hydrogen-ion activity of cooling water is pH 7 or higher. Accordingly, most part of ozone mixed in water by an ozone mixing device, such as an ejector, decomposes and hence it is difficult to obtain ozonous water of a high ozone concentration.
Since the prior art organic deposition preventing system as shown in FIG. 1 needs to supply a large quantity of acid solution to the cooling water, so that the cooling water contains a large quantity of inorganic ions, such as sulfate ions, chlorine ions or carbonic ions, in addition to hydrogen ions. Accordingly, effluence of a large quantity of such cooling water containing a large quantity of organic ions will cause environmental pollution. Furthermore, such cooling water forms scales on, for example, a heat exchanger to reduce the heat-exchanging efficiency of the heat exchanger and corrodes piping.
The organic deposition preventing effect of the prior art pH regulating method using carbon dioxide gas shown in FIG. 2 is not sufficiently high and needs a large quantity of carbon dioxide gas because the pH of all the cooling water must be regulated.
In the prior art microorganism removing system of FIG. 3, most part of the ozone injected into water is consumed by interaction of ozone and reactive substances contained in the water or by the self-decomposition of ozone and hence it is difficult to obtain ozonous water of a high ozone concentration.